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Transport of the essential nutrient phosphorus--primarily in the form of
orthophosphate--into cells and organelles is highly specific. This is
exemplified by the uptake of phosphate or its close analogue arsenate by
bacterial cells by way of a high affinity active transport system dependent on a
phosphate-binding protein; this system is unable to recognize other inorganic
oxyanions and is, moreover, distinct from the one for sulphate transport. The
phosphate-binding protein is a member of a family of periplasmic proteins acting
as initial high-affinity receptors for the osmotic shock-sensitive active
transport systems or permeases for various sugars, amino acids, oligopeptides,
and oxyanions. We report here the highly refined 1.7 A resolution X-ray
structure of the liganded form of the phosphate-binding protein. The structure
reveals the atomic features responsible for phosphate selectivity, either in
monobasic or dibasic form, and the exclusion of sulphate. These features are
fundamental to understanding phosphate transport systems and molecular
recognition of charged substrates or ions in other biological processes.
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